Short wavelength microscopes are devices that produce a magnified image of an object utilizing electromagnetic radiation in the extreme ultraviolet (EUV) and x-ray regime. The wavelengths range from 20 nanometers (nm) to 0.02 nm. These microscopes typically develop image contrast by relying on photoelectric absorption in the test object. Different elements, or structures formed of elements, attenuate the x-rays to varying degrees.
In a typical short wavelength microscope configuration, a focusing element, such as a condenser, is used to concentrate the radiation on the test object. An objective, such as a zone plate, collects the radiation after transmission through the object and forms an image on a detector, such as a charge-coupled device or film. Intervening scintillators are sometimes required, depending on the specific wavelength and detector used.
Absorption-based x-ray microscopy, however, tends to impose certain limitations on the types of structures that can be imaged. Generally, absorption contrast decreases in proportion to the third power of the photon energy of the radiation. This tends to motivate for the use of lower energies, but lower energies may not provide sufficient penetration through the object.
Moreover, absorption-based x-ray microscopy may also fail to provide sufficient contrast between structures within the object of interest when those structures are composed of elements that have similar atomic numbers. Absorption contrast is generally proportional to the fourth power of the atomic number, away from an absorption edge. Absorption contrast x-ray microscopy therefore works well when imaging structures consisting of mostly high atomic number elements, such as gold or tungsten, in a host material consisting of mostly low atomic number elements, such as silicon. It is generally difficult, however, to use absorption contrast to image small structures consisting of mostly low atomic number elements in a test object containing non-negligible amounts of high atomic number elements, such as imaging cracks in dielectrics of a multilevel integrated device.
An alternative to absorption contrast x-ray microscopy is sometimes termed phase contrast x-ray microscopy. Here, the phase shifting properties of the structures within the object of interest are used to create the image contrast between the structures. To utilize the phase contrast, a phase shifting element is typically placed at the back focal plane of the objective to impart a suitable phase shift to the direct beam, i.e., radiation that passed directly through the test object. The focal plane is the plane parallel to the lens that passes through the point at which parallel rays of light meet after being focused by the lens. The phase shifted direct beam interferes at the image plane with radiation that was scattered and diffracted in the test object. Thus, contrast is produced in response to the phase shifting properties or refractive indices of structures within the test object.
Phase contrast x-ray microscopy has some intrinsic characteristics that render it more effective in many types of imaging applications. First, phase contrast is generally significantly larger than absorption contrast in the 0.02–20 nanometer (nm), short wavelength spectral region. As a consequence, exposure time can be substantially reduced. Secondly, phase contrast is inversely proportional to the energy except for a narrow spectrum near an absorption edge. As a result, doubling the energy decreases the phase contrast only by a factor of two while the penetration power increases by a factor of eight. This allows for thicker samples, easing the requirements for sample preparation, or allows for the imaging of samples nondestructively. Moreover, phase contrast between elements is roughly related to the mass density, rather than the atomic number, except for a narrow spectrum near an absorption edge. This enables the imaging of structures comprising low atomic numbered elements alone or in a host materials matrix containing high atomic number elements. It is especially applicable to imaging structures comprising organic compounds, silicon, and/or oxygen, for example.
In the past, the optical trains of phase contrast x-ray microscopes were similar to the trains used for optical frequencies. In a conventional configuration, the scattered light was phase shified relative to the direct beam by, typically, 90 degrees with a quarter wave plate or 270 degrees with a three quarter wave plate that was located at the back focal plane of the objective to retard or advance the phase of the direct beam. There was typically a requirement to attenuate the direct beam so that it had comparable intensity as the collected, scattered signal radiation. This provided higher contrast because complete extinction occurred during destructive interference when the two interfering beams are of the same amplitude.